A diamond appears to shine, but a piece of glass cut to the same shape does not shine.

Question

Accepted Answer

: Diamond has a much higher refractive index than glass, giving it a much smaller critical angle (24° vs 42°). Light entering a diamond easily exceeds this small critical angle at its many faces and undergoes multiple total internal reflections, making it sparkle. In glass, with its larger critical angle, most light simply refracts out through the opposite face instead of being totally internally reflected.